This invention relates to the treatment of gas. In particular, it relates to the treatment of a gas stream comprising hydrogen sulphide.
Gas streams comprising hydrogen sulphide are typically produced as waste products or by-products from many industrial processes. For example, acid gas streams comprising carbon dioxide and hydrogen sulphide are typically produced during oil refinery operations in which sulphur is removed from crude oil. It is necessary to treat such hydrogen sulphide-containing streams before discharging them to the atmosphere so as to reduce or remove all together their content of sulphur containing gases. One well known, widely practiced process for treating a gas stream comprising hydrogen sulphide is the Claus process. This process is based on the reaction between hydrogen sulphide and sulphur dioxide to form sulphur vapor and water vapor in accordance with the equation: EQU SO.sub.2 +2H.sub.2 S=2H.sub.2 O+3S
Sulphur exists in the vapor phase in a number of different molecular species such as S.sub.2, S.sub.6 and S.sub.8 according to the temperature.
The first stage of the Claus process is to burn approximately a third of the hydrogen sulphide in the incoming stream to form sulphide dioxide and water vapor in accordance with the equation: EQU 2H.sub.2 S+30.sub.2 =2H.sub.2 O+2SO.sub.2
This combustion reaction takes place in a suitable furnace and normally air is used a s a source of oxygen for the purposes of combustion. The furnace is designed such that reaction between the sulphur dioxide and hydrogen sulphide can start in the combustion zone and then continue downstream of the combustion zone. At the temperature that is created by the combustion of hydrogen sulphide, it is not possible to convey more than about 75% of the remaining hydrogen sulphide to sulphur by reaction with sulphur dioxide, and typically between 50 to 70% of the hydrogen sulphide is so converted. It is, however, possible to achieve higher percentage conversions in the presence of a catalyst at a reaction temperature in the order of 200.degree. to 350.degree. C. by reacting the remaining hydrogen sulphide and sulphur dioxide. (at such "catalytic" temperatures, the lower the temperature the higher is the percentage conversion that is achieved.) Accordingly, after the gases pass out of the so-called thermal region of the furnace they are cooled to a temperature at which the sulphur that tis form in the furnace condenses. The sulphur is thus recovered. In order to extract further sulphur, the gas stream is then subjected to a plurality of stages of catalytic reaction (between hydrogen sulphide and sulphur dioxide) with the resulting sulphur being separated downstream of each stage and the gas mixture being reheated before the next catalytic stage. By performing 2 or 3 catalytic stages, substantially all of the hydrogen sulphide is removed from the gas stream. In order to remove the last traces of hydrogen sulphide, the gas stream is passed to a so-called tail gas clean-up process of a known kind suitable for handling a relatively dilute hydrogen sulphide stream or is incinerated.
In order to improve the capacity of a Claus plant, it has been proposed to employ oxygen-enriched air to support combustion of the hydrogen sulphide in the furnace. This measure enables the proportion of nitrogen in the gas stream that flows through the plant to be reduced, and its place to be taken by additional hydrogen sulphide. In practice, however, in many plants, the amount of the uprating that can be achieved by this method is limited as there is a tendency for the reduced volume of nitrogen to lead to higher exit temperatures from the furnace than can be withstood by the waste heat boiler associated with the furnace or by the refractory lining furnace. Indeed, the more concentrated (in hydrogen sulphide) the gas stream, the less is the possibility of achieving any significant uprating merely by enrichment of the air in oxygen.
There have thus been a number of proposals in the art for improving the way in which oxygen is used in the Claus process. These proposals can be divided into two categories. In the first category, a fluid, preferably in liquid state, having a higher molar heat capacity than nitrogen is introduced into the furnace so as to moderate the temperature that occurs therein. An example of such a proposal, in which the fluid is water, is disclosed in GB 2,173,780A. The second category involves conducting the non-catalysed reactions in two or more separate furnaces. Accordingly, the heat of the combustion reaction between hydrogen sulphide and oxygen is spread over two or more separate furnaces with the result that less heat is generated in each one individually, making possible in at leas some examples the use of substantially pure oxygen rather than air or oxygen-enriched air to support combustion in each furnace.
Such is the size of a typical Claus plant that to perform such processes using oxygen, it is often preferred to have an on-site oxygen generator rather than to deliver oxygen to the site of the plant from a remote site of oxygen production. For example, a plant producing 400 tons per day of sulphur might typically require in the order of 180 tons per day of oxygen. Oxygen is typically produced in such quantities by the fractional distillation of air at cryogenic temperatures. This method inevitably produces a nitrogen by-product. Sometimes, a use can be found for the nitrogen by-product on the site of the Claus plant. On other occasions, however, there is no such use for the nitrogen by-product.